Image sensor circuits are used in a variety of different types of digital image capture systems, including products such as scanners, copiers, digital cameras, and the like. The image sensor typically includes an array of light-sensitive pixels that are electrically responsive to incident light reflected from an object or scene whose image is to be captured.
The performance of an image capture system depends, at least in part, on the sensitivity of each individual pixel in the sensor array and its immunity from noise. Pixel sensitivity relates to the ratio of a change in the pixel output voltage to the photogenerated charge in the pixel. Noise may be defined as small fluctuations in a signal that can be caused by a variety of sources. Image sensors with increased noise immunity tend to yield sharper, more accurate images in the presence of environmental and other noise.
Improving the noise immunity of each pixel permits a reduction in exposure time, which in turn allows the capture of images at a greater rate. This may allow the image capture system to better capture motion in the scene. In addition to allowing greater frame rate, greater noise immunity may also help to resolve detail in images captured under low light conditions.
Integrated circuit imaging devices typically include an array of light detecting elements interconnected to generate analog signals representative of an image illuminating the device. Within such an integrated circuit, each image-sensing element contained in the integrated circuit usually contains a photosensor, such as a photodiode or phototransistor, as a light-detecting element. In one example, charge is collected or dissipated in accordance with the intensity of light illuminating the photosensor. By storing the resultant charge, an analog signal is thus generated having a magnitude approximately proportional to the intensity of light illuminating the light detecting element during an exposure interval.
During one common mode of operation, each pixel has a photo-sensitive diode, or photodiode, that is first reset by placing a charge across the photodiode. Then, the photodiode is exposed to incident light, which causes the charge stored on the photodiode to be dissipated in proportion to the intensity of the incident light. After a predetermined time period during which the photodiode is exposed to the incident light and charge is allowed to dissipate from the photodiode (i.e., the “integration” or “exposure” time), the charge at the node of the photodiode is read out. This value is the integrated voltage and may be sampled, such as onto a capacitor, by opening a switch or by other means.
After the charge at the node of the photodiode has been read-out, the photodiode is reset by asserting a reset signal to a reset transistor within the pixel. The amount of incident light that is detected by the photodiode is computed by subtracting the integrated voltage level from the reset voltage level.
Due to fluctuations in the power supply voltage, however, the reset level varies between resets. Thus, the noise present in the power supply affects the reset level. Because entire rows of photodiodes are typically reset at the same time, the noise generated tends to affect all the photodiodes in a row. This may result in each row having a different variation in noise that appears as row noise in an image. This noise may be manifest as row-wise coherent noise in an image captured by an array of pixels.
Additional background information is disclosed in U.S. Pat. Nos. 6,133,862, 5,841,126, 6,147,846, and 5,345,266, each of which is incorporated herein by reference.